Heretofore, porous membranes of cellulose derivatives of high perviousness to water, particularly cellulose acetate, have been generally utilized as hydrophilic porous membranes for various forms of filtration and dialysis. Since these cellulose derivatives are deficient in resistance to .gamma. ray, the conventional porous membranes are not usable in the field of medical treatments necessitating sterilization with .gamma. ray. Further, since the cellulose derivatives exhibit poor resistance to such industrial chemicals as acids, alkalies, and organic solvents, the porous membranes have been used under heavily restricted conditions.
To take the place of these porous membranes of cellulose derivatives, porous membranes using as the raw material therefor such fluorine type polymers as polyvinylidene fluoride and ethylene tetrafluoride and consequently excelling in resistance to chemicals and resistance to .gamma. ray have been developed. Since these new porous membranes are hydrophobic, they must undergo a treatment for impartation of hydrophilicity to be effectively used as separation membranes in aqueous type solutions.
As means of treatment for effecting this impartation of hydrophilicity, the method which resorts to the organic solvent (alcohol)-water displacement and the method which relies on coating the surface of pores in the membrane with a surfactant or a hydorphilic polymer have been heretofore known to the art. The former method has a disadvantage that the membrane vested with hydrophilicity by the treatment is wholly deprived of the acquired hydrophilicity once it is dried and the latter method has a disadvantage that the compound deposited by the coating treatment is exuded while the membrane is in use. A method which comprises impregnating a hydrophobic porous membrane with an alcohol, treating the impregnated membrane with an aqueous water-soluble polymer solution, drying the treated membrane, and cross-linking and insolubilizing the residual water-soluble polymer deposited on the surface of pores in the membrane by a heat treatment or an acetalizing treatment (Japanese Patent Publication No. SHO 56(1981)-16,187) and a method which effects the insolubilization of the deposited water-soluble polymer as by means of ionizing radiation (Japanese patent Publications No. SHO 56(1981)-16,187 and SHO 56(1981)-38,333) have been further developed. These methods not only share a common drawback that the treatments involved are highly complicated in process but also entail various drawbacks such as exudation of the portion of the polymer which has escaped insolubilization and decomposition and exudation of the deposited hydrophilic compound during the course of sterilization with the .gamma. ray.
Other chemical treatments with industrial chemicals are now being studied for feasibility. As things stand now, however, no practicable method for chemical treatment has yet been established as because polyvinylidedne fluoride has high resistance to chemicals and because the membrane possesses a special shape of porous texture. It has been known that polyvinylidene fluoride, in a solution of a strong alkali such as potassium hydroxide or sodium hydroxide, undergoes removal of hydrofluoric acid and consequently give rise mainly to a double bond in the molecular unit thereof and entail assumption of a brown color or black color (Journal of Polymer Science, Polymer Chemistry Edition, 21, 3443-3451 (1983)). An idea of imparting hydrophilicity to the polyvinylidene fluoride which has assumed the brown color or black color and incorporated the double bond in the molecular unit thereof by treating the polymer with sulfuric acid has been conceived [Glossary of Manuscripts for Presentation, Japan Polymer Society, Vol. 33, No. 3 (1984)]. This method, however, has as critical drawback that the polymer undergoes a notable loss of strength due to oxidative decomposition of the main chain thereof and the porous membrane managed to be produced at all assumes a brown color. A method of introducing a polar group into the double bond in the molecular unit of the polymer by subjecting to an oxidizing treatment the membrane which has been deprived of hydrofluoric acid in the strong alkali solution is also conceivable. This method, however, entails a disadvantage that the treatment grows in complexity of operation owing to inevitable addition of an extra work and the treatment, when performed under conditions necessary for impartation of thorough hydrophilicity to the surface of pores in the porous membrane, deprives the membrane of strength to a great extent and causes the membrane to turn brown owing to the surviving double bond. All these drawbacks occur because the objects of these treatments invariably have a special form of "porous membrane." If the chemical treatment is carried out with an increased intensity enough for the treatment to proceed even to the interior of the substrate, the double bond formed inside the substrate is not easily subjected to the oxidizing treatment in the second step, with the result that the membrane is caused to turn brown and incur impairment of physical properties. If the chemical treatment is carried out to a lesser extent, it is incapable of thorough impartation of hydrophilicity and consequent fulfillment of the object of the treatment.
In the circumstances, the hydrophilic porous membranes of polyvinylidene fluoride which have reached the practicable level and introduced to the market are colored in brown and, for use in medical treatments, suffer from deficiency in commercial image.
An object of this invention, therefore, is to provide a novel hydrophilic porous membrane of polyvinyldiene fluoride and method for the production thereof. Another object of this invention is to provide a hydrophilic porous membrane of polyvinylidene fluoride having hydrophilicity thoroughly and uniformly imparted therein, excelling in outward appearance, and exhibiting high resistance to .gamma. ray and chemicals and a method for the production thereof. A further object of this invention is to provide a hydrophilic porous membrane of polyvinylidene fluoride capable of easy and expedient manufacture and rich in commercial image and a method for the production thereof.